The present invention relates generally to charging devices and in particular to charging devices which produce a negative corona.
In an electrostatographic reproducing apparatus commonly used today, a photoconductive insulating member may be charged to a negative potential, thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the original document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with a developing powder referred to in the art as toner. During development the toner particles are attracted from the carrier particles by the charge pattern of the image areas on the photoconductive insulating area to form a powder image on the photoconductive area. This image may be subsequently transferred to a support surface such as copy paper to which it may be permanently affixed by heating or by the application of pressure. Following transfer of the toner image to the support surface the photoconductive insulating surface may be discharged and cleaned of residual toner to prepare for the next imaging cycle.
Various types of charging devices have been used to charge or precharge photoconductive insulating layers. In commercial use, for example, are various types of corona generating devices to which a high voltage of 5,000 to 8,000 volts may be applied thereby producing a corona spray which imparts electrostatic charge to the surface of the photoreceptor. A particular device may take the form of a single bare corona wire an array of pins integrally formed from a sheet metal member strung between insulating end blocks mounted on either end of a channel or shield. Another device which is frequently used to provide more uniform charging and to prevent overcharging, is a scorotron which comprises two or more corona wires with a control grid or screen of parallel wires or apertures in a palte positioned between the corona wires and the photoconductor. A potential is applied to the control grid of the same polarity as the corona potential but with a much lower voltage, usually several hundred volts, which suppresses the electric field between the charged plate and the corona wires and markedly reduces the ion current flow to the photoreceptor.
While capable of performing satisfactorily it has been observed that after prolonged use, for example in the process of making about 150,000 copies, difficulties are experienced for both thin metal wire corona electrodes and pin electrode arrays. These difficulties take the form of undeveloped streaks being formed in the copies produced resulting in unpredicatable images. While not wishing to be bound to any particular theory, this is believed to be caused by non-uniform corona generation which in turn is believed to be caused in part by each of several corrosion and erosion mechanisms. The corona causes some sputtering of the metal away from the electrode whether it be a wire or pin electrode which in the presence of oxygen and nitrogen in the air forms metal nitrates which deposit at various locations along the corona electrode. Furthermore, if there is any ammonia in the air white whiskers or powder may also be observed building up at various locations on the corona electrode. These reactions are believed to take place within about 1 millimeter of the electrode and the deposits formed on the corona electrode result in a non-uniformity of subsequent corona generated along the length of the electrode producing hot spots, localized corona, in the location of the deposits. It is believed that these hot spots tend to create a higher electrostatic field resulting in non-uniform charging. Furthermore, on a clean corona electrode, the hot spots tend to move along its length and are of a lower intensity than after an extended period of use. As corona electrode ages, the hot spots become more intense and become fixed in location thereby accelerating further corrosion at their locations resulting in increased non-uniformity of corona and thereby non-uniformity of charging of the imaging surface. In addition in the pin-type electrode, the sputtering of metal around the pin results in a collar of deposits which build up around the pin and which eventually results in a periodic non-uniformity such that every other pin is dominant. This results in an as yet unexplainable inactivation of corona generation at every other pin.
Previous attempts to minimize the difficulties associated with the above-described erosion and corrosion processes have included physically periodically wipping the corona electrode with a cloth or foam pad. Alternatively, the corona electrodes have been coated with gold. This is effective although expensive and difficulties are frequently experienced in the adhesion of the gold to the corona electrode since the gold tends to flake. Alternatively, fewer difficulties are experienced with platinum wire as the corona electrode which has a lower rate of degradation.